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Behavioral Ecology and Sociobiology

, Volume 49, Issue 4, pp 260–265 | Cite as

Offspring sex ratio in relation to maternal age and social rank in mountain goats (Oreamnos americanus)

  • Steeve D. Côté
  • Marco Festa-Bianchet
Original Article

Abstract

In polygynous mammals, high-quality females may increase their fitness by producing a high proportion of sons. During a 9-year study of marked mountain goats (Oreamnos americanus), we assessed the relative effects of maternal age, social rank and reproductive status on offspring sex ratio. The sex ratio of kids in the population did not differ from unity (75 males, 85 females). The proportion of female kids decreased markedly with maternal age. Young females (≤6 years old) produced approximately 70% daughters, while old females (≥10 years old) produced about 25% daughters. The proportion of females born did not vary with maternal social rank when accounting for mother's age. Weaning success one year did not affect the probability of producing a son or a daughter the following year. Kid sex was independent of the sex of the kid produced by the same female the previous year. Because the proportion of daughters produced decreased dramatically with age, and because males appear more costly to rear than females in sexually dimorphic ungulates, our data suggest that the ability to provide maternal care may increase with age in mountain goats. Accepting that older mothers are better mothers because of higher social rank and experience, our data support the predictions of the Trivers and Willard hypothesis.

Age Dominance Mountain goat Oreamnos americanus Reproductive success Sex ratio 

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Copyright information

© Springer-Verlag 2000

Authors and Affiliations

  • Steeve D. Côté
    • 1
  • Marco Festa-Bianchet
    • 1
  1. 1.Groupe de recherche en écologie, nutrition et énergétique, Département de biologie, Université de Sherbrooke, Sherbrooke, PQ, J1K 2R1, CanadaCanada

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